The present invention relates to electromagnetic interference (EMI) filtering apparatus and methods and, more particularly, to active EMI power line filtering for power devices using switching converters.
Traditional passive input power line EMI filters utilize series inductors and shunt capacitors to provide the filtering action which reduces the flow of interference currents from a source within the equipment of concern back onto the power bus providing power to that equipment. The inductors may be single winding, to predominantly reduce differential mode currents, or wound with multiple conductors to predominantly reduce either differential mode or common mode interference, depending upon the connections of the windings. If the windings on a multiple winding inductor are connected in such a way that the power frequency flux contribution from each winding is additive, then net differential mode inductance is present, and higher frequency differential mode current will be attenuated. If the windings on a multiple winding inductor are connected in such a way that the power frequency flux contributions from the windings are opposing, and tend to cancel, then differential mode inductance will be absent, but common mode inductance will be present. EMI power line filter inductors should be designed to attenuate the type(s) of interference, differential mode or common mode, which are present.
The capacitors in a traditional passive power line filter may be connected line-to-line, to reduce the amplitude of differential mode currents flowing back onto the input power bus, or line-to-ground, to reduce the common mode interference current. The capacitors should be connected to treat the type of interference present.
Power conversion equipment utilizing switching converters generally produces harmonics of the switching frequency which are impressed upon the input power leads. The purpose of input power line filtering is to reduce the level of these harmonics to an acceptable level, such as imposed by MIL-STD-461E (Military Standard Specification) or DO-160E (Radio Technical Commission for Aeronautics, RTCA standard). Depending upon the EMI specification imposed, either the voltage harmonics or current harmonics may be measured. There may also be power line contamination from rectifier noise, and from the harmonics of clock frequencies used by digital circuits. The proper EMI filter for a given application will include appropriate levels of both differential mode and common mode filtering, over the respectively required frequency ranges, to meet EMI specification requirements with an adequate safety margin.
Inductors for differential mode filtering are typically on the order of 10 to 500 microhenries. Inductance component values generally decrease as inductor current rating increases. These inductors allow the power frequency (e.g., DC, 50 Hz, 60 Hz, 400 Hz, or variable frequencies from 360 Hz to 800 Hz) to pass unimpeded, and perform as high series impedances at higher frequencies. At the power frequency, a good practice is to limit the total filter inductor differential mode inductive reactance to one percent of the magnitude of the filter load impedance. For the filter inductor to present a relatively high series impedance to high frequencies, it is important to control the inductor parasitic winding capacitance. This capacitance, which represents the net effect of the turn-to-turn capacitance and the capacitance from inductor input lead to output lead, is generally on the scale of tens of picofarads. The intentional inductance and unintentional parasitic capacitance will form a parallel resonant circuit. Below resonance, the inductor performs as an inductor. Above the resonant frequency, the inductor performs as a series capacitor. Inductor saturation is also a concern; an excessive number of ampere-turns on a given core will cause the effective permeability to decrease, resulting in less than desired effective inductance.
Differential mode filter capacitors are generally on the order of 1 to 50 microfarads. These capacitors present a high impedance to the power frequency and other very low frequencies, but present a low impedance to much higher frequencies. As a general guideline, to prevent untoward power frequency effects, the capacitive reactance at the power frequency due the total effective differential mode capacitance should be at least one hundred times the impedance magnitude of the filter load. The low shunt impedance due to the differential mode capacitance serves to return differential mode interference current found on one input power lead to the interference source, via a companion power lead. For these differential mode shunt capacitors to work, the capacitors must display a low impedance at the interference frequency of concern. Tending to spoil (i.e., increase) this low impedance is the unintentional stray inductance due to capacitor construction or capacitor lead length (generally on the order of 25 nanohenries per inch of total lead length). This inductance forms a series resonant circuit with the capacitance. Below resonance, the capacitor acts as a capacitor, with decreasing impedance magnitude with increasing frequency. Above resonance, the capacitor acts like an inductor, with increasing impedance magnitude as frequency increases. A properly designed and installed differential mode capacitor will display insignificant stray inductance over its frequency range of concern.
The predominant shortcomings of passive filter elements are summarized below, to provide insight for desirable components to possibly replace with active filter circuits:                Differential mode inductors—Tend to be large and heavy, especially for high current input power lines. Tend to have rather low self-resonant frequencies (lower than 5 MHz);        Differential mode capacitors—Size may become a problem for higher voltage input power lines. Tend to work best at lower frequencies (lower than 5 MHz) due to lead length inductance;        Common mode inductors—Size may become a problem for high current power lines. Tend to work best at lower frequencies due to low self-resonance (lower than 5 MHz); and        Common mode capacitors—Are generally very small components, and can be very effective higher frequency filters (higher than 10 MHz). Performance is limited, especially at lower frequencies, due to personnel safety limits on capacitor value. Increased high frequency performance can be gained by using surface mount (better) or feedthrough (best) configuration. Feedthrough configuration can increase volume required, and complicate manufacturing somewhat.        
The EMI related devices and provisions as a part of a power electronics equipment may occupy up to 30% of the total weight and volume. Therefore, there is a need in the modern power electronics aerospace industry for new approaches for improved performance while occupying less total weight and volume as compared to conventional passive filters.